Automatic flight control and approach and landing system for aircraft



May 22, 1951 E. F. sAxMAN, JR

AUTOMATIC FLIGHT CONTROL AND APPROACH T AND LANDING SYSTEM FOR AIRCRAFT 5 Sheets-Sheet l Filed Aug. 9, 1947 May 22; 1951 E F SAXMAN, JR 2,553,983

AUTOMATIC FLIHT CONTROL AND APPROACH AND LANDING SYSTEM FOR AIRCRAFT Filed Aug. 9, 1947 3 Sheets-Sheet 2 POWER CON TROI.

BY @MTM A TTORNE YS E. F. SAXMAN, JR AUTOMATIC rFLIGHT CONTROL. AND APPROACH AND LANDING SYSTEM FOR AIRCRAFT May 22, 1951 3 Sheets-Sheet 3 Filed Aug. 9, 1947 JOIPZOO| ZEDP INVENTOR:

lfydwmlfaxmmgcfg BY @aI/,Q ATTORNEYS.

Patented May 22, 1951 UNITED STATES PATENT OFFICE AUTOMATIC FLIGHT CONTROL AND APPROACH AND LANDING SYSTEM FOR AIRCRAFT 16 Claims.

The present invention relates broadly to night control of aircraft and to an approach and landing system therefor and particularly to the continuous automatic control of aircraft both as a part of flight control and in approaches for landings. Within the broad eld of aircraft approach and landing the present invention includes means for the control of the position of an aircraft in a vertical direction relative to an externally generated beam or glide path from a position relatively remote from the airport until contact is made With the runway and a process for holding an airplane on such glide path.

Various methods have hitherto been proposed for accomplishing the safe landing of aircraft under restricted conditions of ceiling and visibility. All such systems must take into account the fact that the pilot during some or all of the approach and landing procedure lacks visual contact with the ground and must therefore substitute some other means for the normal visual reference in making such approaches and landings. One of the oldest of such systems, and one still Widely in use, involves the employment of a corrected sensitive altimeter to give an indication of altitude above the landing field and the use of a radio beam to give a directional line of reference leadingr to a point near the airfield. Using this system, the pilot maintains his position along the radio directional beam and gradually reduces altitude to a predetermined figure over a known point along the directional beam. After crossing this4 point additional altitude is lost at a predetermined rate for a predetermined time so calculated as to bring the airplane over the desired landing field at a safe minimum altitude. The pilot then lands the airplane by visual reference to the ground if he has broken out of the overand can see the ground, or, if visual ground contact has not been made, he immediately ascends to a safe altitude and repeats the procedure. This system is subject to a number of disadvantages and serious limitations. In addition to being time consuming and diflicult under adverse conditions, the system described above cannot be used under conditions of very low ceiling and visibility. The reason for this lies in the fact that the final stages of the approach and the actual landing must be made with the pilot having visual contact with the ground for an appreciable time prior to making contact with the runway. At most airports the minimum safe altitude for establishing visual contact is between 8Go and 4.00 feet. Consequently this system cannot be safely used when the ceiling is lower than such minimum safe altitude. An added disadvantage arising from the use of this system is the time consumed and the general delay which results from the congestion caused when one or more approaches are missed by incoming aircraft. The time required for each individual approach is such that the total waiting time for the last ship in the stack may be as long as three hours. This is obviously impractical, inefficient and hazardous.

In view of the diiliculties referred to above, other systems have been proposed for guiding aircraft to safe landings under conditions of low ceiling and visibility. One such system Was developed by the armed forces during the late War and involvesthe use of radar to guide the pilot to a safe landing by radio instructions issued by operators stationed on the ground. This is the so called Ground Control Approach System known as GCA. While this system can be used successfully under conditions which would prohibit the use of the earlier instrument system described above, it is still subject to serious disadvantages which are inherent in this type of system. Not the least of these disadvantages is the fact that the system cannot be made to operate automatically since it is dependent on the following by the pilot of radio instructions which he receives orally from the ground. There is also the difficulty of absolute certainty of identification of the aircraft being controlled, particularly under conditions Where several airplanes are in the same general vicinity. Itis also dependent on the pilot doing exactly what he is told to do.

Still a third system which has been proposed involves the use of a radio beam or glide path which is projected from the airport at a predetermined angle to the horizontal and a directional or localiser beam projected in a vertical plane. Various systems have been proposed for automatically maintaining aircraft on such a glide path. One such system involves using the elevators to bring the aircraft back to such glide path on vertical displacements therefrom. This system has not been successful under all conditions because a minimum air speed must be continuously maintained in order to avoid stalling the airplane. This is usually accomplished by attempting to maintain a constant air speed Vwhich must be calculated because of the varying Weight of the aircraft. The principal diiculties inherent in such a system, aside from the troublesome calculations required, are that the minimum air speed which must be maintained is necessarily excessive as of the time of actual landing, and the action of bringing about a nosedown condition in order to follow the glide path is fundamentally dangerous when the aircraft is at low altitudes since a dip in the glide path will result in a nose-down condition when the airplane is too near the ground to permit of recovery. Moreover, for high wing loadings the minimum air speed becomes too high to be practical since the minimum speed must take into account the necessity of making turns.

The present invention is directed to overcoming the diculties of the systems referred to above and, as will be seen from the description which follows, operates on a principle entirely different from any other system heretofore proposed.

In addition to the above, the present invention is also directed to providing means for accornplishing control of aircraft while in flight. With the means now available in the art, it is not possible to establish and continuously maintain a desired angle of attack, although it is recognized that to-obtain maximum range an airplane should be flown at a certain constant angle of attack and that likewise maximum endurance in the air and maximum efficiency can be obtained only by maintaining predetermined angles of attack. This problem is complicated by the fact that attitude and/'or power requirements for maintaining a given altitude at a constant angie of attack are continuously variable as the load decreasesor increases and in normal operation there is a continuous decrease in load as fuel is ccnsurned. The present invention provides means for selecting a desired angle of attack and also means for manually or automatically maintaining flight at the desired angle of attack independently of changes in load or attitude.

Therefore, one object of the present invention is to provide a combined night control and approach and landing system for aircraft.

A further object of the present invention is to provide an approach and landing system for bringing an airplane to a safe landing under conditions of restricted ceiling and visibility.

A further object of the present invention is to provide an automatic approach and landing system'which is inherently ystall proof.

A still further object of the present invention is to provide a system for night control designed to maintain automatically any desired safe angle of attack.

A still further object is to provide means and process for manually controlling an airplane to maintain its position on an externally generated glide path.

In the description of the present invention which follows, reference will be made to the accompanying drawings in which:

Fig. l represents diagrammatically the essential controlled aircraft elements and the controlling means of the present invention.

Fig. 2 is a more detailed showing of the elements ofthe `invention shown in Fig. l..

Fig. 3 is Aa circuit diagram of the elements showninFig. 1 and Fig. 2 and an alarm system connected thereto.

Fig. 4 is a diagrammatic showing of the reversing mechanism `of circuit '6.

In the following description of the present invention the term angle of attack will occur frequently. In the past there has been some difference in terminology as between the United States. and other countries such as Great Britain with respect to this term and it is therefore here dened in the sense in which it will be used throughout this specification and the appended claims. Where so used, angle of attack is intended to mean the angle between the chord of the wing of the aircraft in question and the direction of the relative wind in a plane pci'- pendioular to the lateral axis of said wing.

In describing the present invention the approach and landing systems will be iirst described and the fiight control system will be separately described at a later point in the present specification. It is to be noted that both systems en ploy the saine general means although in a different way. The outstanding difference between the two is that the approach and landing system involves controlled changes in angle of attack at constant attitudes whereas the night control system operates to change the attitude while holding a constant angle of attack.

The approach and landing system is designed to operate in conjunction with a glide path beam and a localiser beam generated at the landing area in ways well known to the art. One such system which may be used is designated as the SCS-5l, or ILS, system, the transmitting components of whch are manufactured by Federal Radio Laboratories, Inc. It consists essentially of a glide path transmitter designed to propagate a high frequency glide path beam of limited vertical dimensions inclined upwardly from the landing area at a predetermined angle from the horizontal, and a localizer transmitter designed to transmit a directional beam at a much lower ireduency in a vertical directional 'beam of ited lateral dimensions. The desired track in space for the airplane approaching for a landing is the intersection of the inclined glide path beam and the vertical localizer beam. All points removed from such intersection have signal charact tics which differ from the intersection characteristics. While the above described system is suitable for use in connection wtih the present invention, it will be understood that other systems may be used. The present invention is directed in its approach and landing aspect primarily to maintaining the aircraft on the desired track and is independent of the method used for establishing such track except for the provision of means responsive to the elements thereof.

It is also possible to use the present invention on other types of ranges including controlled omni-directional ranges in which case the lateral connection may be handled in a number of ways including the use of electronic computers.

In Fig. l of the drawings, a glide path generated as described above is shown diagrammatically at l and a glide path receiver adapted to being tuned to the frequency of glide path indicated at 2. Glide path receiver 2, together with the rest of the equipment hereinafter described, is carried aloft by the airplane. It consists of a standard and well known glide path receiver manufactured by `Federal Radio Laboratories, Inc., under the designation AN/ARN-. Its function is the reception of the glide path signal and the production of an output signal which is continuously and automatically varied upon departures of the aircraft in either an up or down direction relative to the glide path. This variable output from receiver 2 actuates the horizontal needle 3l of the cross pointer indicator i which is a standard instrument normally used in conjunction with the AN/ARN- receiver.

The horizontal needle 3l of indicator 4, operating through speed network 5 and reversing network 6, brings about changes in the operation of angle of attack selector motor 1. These changes at angle of attack selector 'I result in changes in angle of attack network 3 and in the position of needle I2 of angle of attack indicator I0. Changes are also made in angle of attack network 8 in response to movements of angle of attack device II which movements are also used to produce changes in the position of needle 9 of indicator I9. Power adjuster I3 operates in response to the output from angle of attack net- Work 8 and brings about control of the operation of reversible motor I4 which in turn makes adjustments through power control I5 to vary the power output derived from the power plant of the airplane. One Way of accomplishing such power changes is through adjustments of the throttle settings. As hereinafter described, power adjustor I3 is also operated to increase the power output from the power plant when the angle of attack of the airplane as measured by device I I and needle 9 reaches a predetermined critical value. This action to increase power at a predetermined critical angle of attack is shown diagrammatically by dotted line I5.

In Fig. 1 there is also a diagrammatic showing of a localizer beam I'I and elements responsive thereto for maintaining the airplane on the localizer beam. This is accomplished by the action of localizer receiver I8, the output of which actuates the vertical needle of indicator 4. It also is used to adjust the turn control mechanism 29 of automatic pilot 2l in order to hold the airplane on the localizer beam. The localiser receiver I8 may be the standard well known RC-103-A manufactured by Federal Radio Laboratories, Inc. Automatic pilot 2! may be the well known auto-pilot manufactured by Minneapolis- Honeywell Regulator and Control Company. In addition to its other functions it may be used to establishv and maintain a desired attitude through the action of the dive control mechanism forming a part thereof. Such desired attitude may be maintained independently of changes in other variables such as the throttle settings. Automatic pilot 2! operates in a well known manner through motors 22, 23 and 24 to control elevators 25, ailerons 26 and rudder 2l respectively. The above described system for maintaining an airplane on a localizer beam is well known in the art and its construction and use is therefore merely shown diagrammatically in the drawings. In addition to receiving impulses from localizer receiver I8, turn control 29 of automatic pilot 2l is also connected to the output from angle of attack device II through indicator i3 in such a way as to bring about a decrease in the rate of turn or to stop all turn at a predetermined critical angle of attack. This action is shown diagrammatically by dotted line 23 in Fig. l.. Likewise the dive-climb control 29 of automatic pilot 2l is connected to the output of angle of attack device II through indicator I9 in such a way as to depress elevators 25 in the event a predetermined critical angle of attack is reached when the airplane is in straight night. This connection is shown diagrammatically by dotted line 3D in Fig. 1.

In Fig. 2 the operative relationships between the various elements shown in Fig. l are represented in more detail, although it will be understood by those skilled in the art that other means for bringing about the desired effects may be substituted for those shown. The horizontal pointer 3l of cross pointer indicator I is shown as carryu ing a contact point 32 which slides along variable resistances 33 and 34 which are connected to one side of the line at L2. Pointer 3I is connected to one side of angle of attack selector motor 'I through line 39. Therefore, the speed of motor 'I is Varied with changes in the total resistance of that part of network 5 which is momentarily in circuit between L2 and motor 'I. The result is that motor 1 operates at faster speeds as the aircraft departs further from the glide path. At the horizontal position of pointer 3I, motor 'I is inoperative. A second contact point 40 is carried by pointer 3l and this contact point engages variable resistance 4I when pointer 3| is in the down position. Contact 43 engages variable resistance 42 when pointer 3l is in the up position. Contact `point 49 is electrically insulated from contact point 32 and the main portion of pointer 3| by insulation 43. Through line 44 contact point 45 is connected to one side of battery 45 and to line 5Fl through switch |35. The other side of battery l5 is connected to one side of the coil 48 of solenoid 49. The other side of coil 49 is connected to resistances 4I and 42 through switch IlI, line 59 and thence to point 4S. Solenoid 49 operates reversing switch 5I which is connected to shunt winding 52 of reversible motor 'I. The other side of reversible motor 'I is connected to the line atLI through switch I25.

Angle of attack selector motor 1 is mechanically connected to the movable arm 53 of variable resistance 54. Movable arm 53 is also adjustable manually independent of the action of motor 'I through manual angle of attack selector 55. Movable arm 56 of variable resistance 51 is mechanically connected to angle of attack device II and is responsive to movements thereof. Angle of attack device II is also shown as inechanically connected to angle of attack indicator Ill to actuate pointer 9. This connection operates pointer 9 in such a way as to indicate the actual angle of attack of the airplane. Motor 'l is shown as mechanically connected to pointer I2 of indicator I9. Therefore changes in the position of pointer I2 reflect changes in the reuuested angle of attack established by motor 1. Variable resistances 54 and 51 are connected in series with battery 58. This network, comprising angle of attack network 8 of Figs. 1 and 3 is connected to milliammeter I3 which corresponds to power adjustor I3 as shown in Fig. 1. Needle 59 of milliammeter I3 is connected directly to one side of motor le. The other side of motor Ie is connected through switch H13 to one side of the line at LI as shown. Sector 6I] is mounted to the left of needle 59 and is connected to the other side of the line at L2 as shown. Sector 3l is mounted to the right of needle 59 and is connected to one side of coil 52 of solenoid 33. The other side of coil 62 is connected to the battery 55 and theother side of battery 64 is connected to needle 59 through lines G5 and 65. Solenoid 53 is provided for operating reversing switch 5l which is connected to the shunt winding 53 of reversible motor I4. The movable elcment 59 of solenoid 53 carries a contact member 'I5 which is provided for making contact between points 'II and 'I2 in line L2 when solenoid 63 is energized and which breaks contact between points 'li and 'I2 when solenoid 52 is deenergized. Motor I3 is mechanically connected to power contr-ol I5 through manual power control means 'I3 which. is operative independently of the action of motor I4. An alarm switch 'I4 is shown as a part of angle of attack indicator I9 and acts to close contacts 'I5 and 'I6 when arm 9 on which it 7 is mounted reaches a predetermined critical value for the angle of attack. This has the effect of energizing solenoid 33 through the connection of lines 11 and 18 of Fig. 2 and lines |89 and |94 of Fig. 3. The result is an immediate increase of power at power control l and a consequent decrease in angle of attack. .er second alarm switch is provided at 19 which operates to connect points 36 and 8| of Fig. 2 when pointer 9 reaches a predetermined critical value of angle of attack. Point 89 is connected to the L2 side of the line and point 8| is connected to the mov-- able pointer ||2 of turn gyro |I| through point 82, switch i4 and line I i3 as indicated in Fig. 3.

Fig. 3 comprises the electrical circuits involved in the operation of the elements shown in Figs. 1 and v2 and in addition includes the electrical circuits for alarm means designed to automatically bring about the proper changes in the controls of the airplane when ,a predetermined dangerous angle of attack is reached. In Fig. 3 the two sides of the main power line are shown at Ll and L2. In order to aid in the understanding of the description of the operation of the present invention which follows, the various circuits inu volved in Fig. 3 will now be traced. Starting with speed network 5, resistance elements 33 and 34 are connected to the L2 side of the line. The speed network circuit may then be followed through resistance 33 to the movable contact point 32. From resistance 33 the circuit flows through pointer 3| when pointer 3| is in the vdown position in Fig. 2 which corresponds to a swing to the left in Fig. 3. The circuit then goes through line 39 to the left side of motor 1. The other side of motor 1 is connected to the LI side of the line through the line 85 when switch |25 is in its normally closed position. Similarly, when needle 3| takes the right hand position, the circuit passes from L2 through resistance 34 to contact point 32 and thence through pointer 3| and line 39 to the left side of motor 1. Dotted lines 33a and 34a are intended to represent a variati-on in the position of contact point 32 along resistances 33 and 34 respectively responsive to movements of pointer 3| as shown diagrammatically in Fig. 2,

Reversing network 6 is independent of the lmain power line and has a separate current source consisting of battery 45. tery 45 is connected to coil 48 of solenoid 49 through line 88. The other side of battery 45 is connected to the other side of coil 48 through line 44 to the point 49 which is carried by pointer 3| of Fig. 2. When pointer 3| is in the down position, corresponding to a swing to the left in Fig. 3, contact is made with line 93 and the circuit therefore flows through variable resistance 4| and thence through line 53 and switch |40 to the other side of coil 48. When pointer 3| is in the up position, contact point 4!) moves to the right and the circuit flows through line 9| and thence through resistance 42, line 59 and switch |49 to the coil 48 of solenoid 49. Dotted lines 41a and 42a are intended to represent a variation in the position of contact point 4|! along re- Vsistances 4| and 42 respectively responsive to movements of pointer 3|. Switch |35 is provided connecting battery 45 with coil 43 through lines |39 and 50.

Solenoid 49 is either energized or deenergized depending upon the total resistance established between the battery 45 and line 53 and the action of switches |35 and |48. The action of solenoid 49 is such as to throw reversing switch 5| in one One side of bats direction on energization and in the opposite drection on deenergization. This results in a reversing of the direction of rotation of motor 1 with a consequent change in requested angle of attack.

Upon rapid changes in position of the aircraft relative to the glide path it is desirable to provide means for changing the direction of rotation of motor 1 prior to the time when the aircraft returns to the glide path. By thus changing the requested angle of attack before the inertia of the aircraft carries it beyond the glide path, a smoother operation is obtained than would otherwise be the case. This may be accomplished in a number of ways. One device for accomplishing this is illustrated in Fig. 4 in which there is shown a slide member 46 which is operated by a mechanical connection to the glide path coupler 3. Connections of this sort are known to the art and need not be described here. Slide 43 is guided in slot |38 formed by flange members |3|. Channel |32 may be suitably lubricated to provide ease of operation. Mounted in slide 43 is an insert |34 constructed of Silinite Arm |35 which consists of a fiat spring carries a contact element |38 which rides on the surface of Silinite body |34 with a pressure of about one ounce maintained by spring i35. Silinite is a well-known combination lubricant and resistance composition made by Watson Stabilator Company of Philadelphia, Pennsylvania. Pivot support |29 is provided for spring |35. Spring |31 is attached to arm |35 and holds it in a normally open position with respect to contact |33 which is connected through line |39 to one side of the battery 45. A similar slide, not shown, is used to operate switch |40 except that this switch is held in a normally closed position by a similar spring arrangement. The Silinite bar |34 has the characteristic of offering greater resistence to travel when it is moving fast relative to the contact element |36 than when moving slowly. Consequently, on a rapid motion of slide 46 to the right in response to a rapid downward motion of the airplane toward the glide path the contact end of arm |35 is moved to the left and makes contact with point |38. Since arm |35 is connected to one side of the solenoid coil 48 the resultant contact energizes solenoid 49 and operates reversing switch 5|. The result is that motor 1 is rotated in a direction calling for less increase in requested angle of attack. Similarly, a rapid motion to the left of the slide operating switch |48 when the aircraft moves rapidly upward toward the glide path results in an opening of switch |43 and this brings about a reversal of motor 1 through the deenergization of solenoid 49. This results in a rotation of motor 1 in a direction calling for less decrease in angle of attack.

Angle of attack network 8 is also independent of the line and has a separate source of current comprising battery 58. One side of battery 58 is connected through line 92 to one of the terminals of milliammeter I3. The other terminal of milliammeter i3 is connected to resistance 54 through line 93 and movable arm 53. The circuit is then completed through line 94, movable arm 56, variable resistance 51 and line 95 to the other side of battery 58. Variations in the eifective resistance at 54 and 51 are made through the action of motor 1 and movements of angle of attack device |I respectively.

The circuits for operating reversible motor |4 maracas.

will'now bedescribed. One side'ofmotorf- |44is connected directly tol the LI side ofthe line.

throughline 96Y andswitch |03. Theother side of motor I4 is connected to the L2 side of the lineithrough pointer 59, line 91 and switch |05. This connection is made when pointer 59,k swings to. the left to makencontact with point; 98 which lies on` segment E ofFig.2. When pointer59- is inthe right hand position makingv contact with point 99 kwhich lieson segment 6| of Fig. 2, the.

leftshand side of motor |4 isconnected to the L2 side of: the line through line |00, switchA 10, line |02, line 91. and switch |05. Switch is closed when pointer: 59 is in the right hand position in response tothe action of solenoid 63 which is operated through a circuit independent of the line and having a battery source 64 connected directly to one end of solenoid coil 62. TheV other end of coil 62 is connected to the other'sideo?` thev battery 64 through line |04, point 99, pointer 59,.line, 66and line |00when pointer isin the right hand position. Whenpointer is outof contact with point 99 and switch 14 is in its normally open position, coilr 62 is not connected to both sides'of battery 64 and solenoid 63 is therefore deenergized. In this position the circuit between points 1| and 12 is broken. Reversn ing switch 61 is operated in response'to energization and deenergizationl of solenoid 63. operation results in a reversing of the direction of rotation voi motor I4V by reversal of connections to shunt winding 68. `Landing switch |05 is normally closed but is opened when the airplane make contact with the ground. Means lfor mounting and operating such aA switch on landing are shown. in my U. S. Patentk #2,193,077 and therefore are not shown inthe present application. Switchf14` is adapted to.A close thecircuit between points and 16 when a predetermined critical angle of attack is* indicated by pointer 9 ofvFig. 2. This results in energizingy solenoid G3 since-one side of coil 62 is thereby connected toone side of battery 64 through line |04, point 16, switch 14,- point 15 and line |00. The other side of coil 62 is permanently connected to the otherfside of battery 64. When 63 isv thus energized, motor i4. is caused'to run in a direction toincreasepower through power control I5 provided landing switch |05 is in its normally closed position. Switch |03 is provided for completely disconnecting motor I4 from the Ll side of the line.

Turn control motor |06 is connected to the LI side 0I" the line through lines |01 and 85.

The other side of motor |06 is connected through line |08 to segment |09 and segment HQ of turn gyro When the pointer I2 of turn gyro moves either to the left or to the right, contact is made to the L2 side of the line through line H3 provided switch 19 is closed/and provided also that landing switch ||4vis in its normally closed position. Landing switch |4- operates Vin the same way as landing switch |05. Switch 19 is closed only when pointer 9 of angle of attack indicator' I0 reaches a predetermined critical value. The reversing circuit for motor |06 has a separate source of current comprisingbattery l5, one side of which is connected to one side of' coil ||6 of solenoid ||1. The other side of battery l5 is connected to the other side of coil ||6 through line H8, switch ||9 and line |20. Switch H9 is closed when pointer ||2 swings to the left and is open when pointer |2\ is centered or swings to the right. Energization of solenoid ||1 operates reversing switch |2| and re-f This v 10 sults inachange -in the direction of rotation. of motor |06 during the periods when motor |06 .is operative which will beonly whenl pointer H2 is positioned either to the right or left` of center at atime'when switches 19 and ||4 areboth closed. Operation of motor |06 causes neutralizing changesin the positionof turn control 20-which.

is astandardinstrument which operates inV such'.

a way as to center pointer H2 when the airplane is in straight flight and to swing pointer ||2yei ther to the. left or to the right when the air plane. is turning.

Having describedfmeans embodyingmyinvem tionIgwill. now describe the operation' of such.

meansin carryingA out thev objectsof my invention. This description divides itself logically into two sections, the'nrst having todo with the use. of my invention as an approachzand landing.

systein, and', the secondY havingl todo withzthe use of.:my invention `in carrying out lightmron-l trol., These will be separately discussedzin4 the order mentioned.

TheA operation of holding, the, airplane on the localizerbeamis well known in the ,art andtherefore requires .no further description. It issulie cient.. to state thatL ther `necessary coordinated turnstoy holdV the localizenbeam are` made automatically by4 theautomatic pilot ,2|. in` response to the signals coming from localizer receiveral. Theoperation ofthe present inventicnvto automatically put the airplanel on the glide path and hold it there is notknown to the artandwilly therefore be described inv detail.

The output from glide path receiver. Zoperates cross pointer indicator 4 and, in away known: to the art, the horizontalpcinter; 3|. is operatedA in such a way that circle |24 isv above pointer. 3| when the airplane is abovevthe glide` path and below pointer 3| when the, airplane is below they glide path. This. is accomplishedV by moving pointer 3|'in response to the glide path signals While circle |24 remains stationary.- with respect totheindicator face. This operation of 'pointer` 3| is well knowntouthe-art. and is therefore not here describedin. detail. As shown infFig. Zandy ances 35er 36 which are also in the speed control circuit'v and which are independently variedinv response to the actionof rate'gyros.' 31 and'38 respectively.r These rate gyros operate to increasetheresistance at 35 andv 36 as the rate of approach of the airplane to the glide path increases. This results'in the introductionof anl'i other factor to decrease the speed of rotation of motor 1 as the rate of approach of the aircraft to the glide path beam increases.

The direction of rotation of angle of attack selector motor 1 is controlled by the operation of solenoid 49 which in turn operates in response to changes in the total resistance in reversing network 6. As the airplane approaches the glide path from above, the effective resistance of variable resistance 4I becomes less as pointer 3l moves upward. When the airplane approaches the glide path from below, the eiective resistance at variable resistance 42 becomes greater as pointer 3I moves downward. By choosing the proper values for resistances 4I and 42, solenoid 49 is made to operate to reverse motor 1 as desired since coil 48 will be sufficiently energized only at total resistance values below a predetermined critical total. On rapid changes of the airplane relative to the glide path an advance reversal of motor 1 is obtained through the action of switches and |40 as described earlier in this specification.

The combined action of speed network 5 and reversing network 5 on reversible motor 1 results in a controlled change in variable resistance 54 when motor 'I is caused to run. This change has the effect of establishing a requested angle of attack. This requesting action which is initiated through changes in variable resistance 54 is brought about by the fact that milliammeter I3 is So selected that pointer 5S is centered at a predetermined current value, such for instance as milliamperes, and battery 58, resistance 54 and resistance 51 are so chosen as to produce a 50 milliampere current when movable arms 53 and 55 are approximately at the mid points of resistances 54 and 51. Assuming that needle 59 is centered, any change in resistance 54 will result in an increase or decrease of current from angle of attack network 8 to milliammeter I3, depending on whether resistance is subtracted or added. This will result in a displacement or" pointer 59 to the right or to the left. Such displacement can be corrected only by an equal and opposite change in variable resistance 51 so long as the initial change at resistance 54 is maintained. Since changes at resistance 51 are brought about only by changes in the angle of attack of the airplane acting on angle of attack device II, the net effect of the change made by angle of attack selector motor 1 is to request a new angle of attack for the airplane and this new angle of attack is achieved by adjustments to power control I5 through the action of motor I4 which in turn is controlled by power adjuster I3. However, as the airplane returns toward the glide path the requested angle of attack will change and therefore the action will be to adjust power to match a continually varying requested angle of attack.

During the approach and landing process the attitude or pitch of the airplane is maintained `at a predetermined desired angle through the operation of dive control 29. This value may be changed as the airplane proceeds down the glide path but at any one instant of time it is held at a xed value independently of changes occurring in the angle of attack of the airplane which chang-es are primarily brought about by changing power input. With the attitude thus fixed, changes in the angle of attack provide an effective means for holding the airplane on the glide path since a suiciently large increase in angle of attack will bring the airplane back down to the glide path from above and a sufficiently large decrease in angle of attack will bring the airplane back up to the glide path from below. Under this condition of fixed attitude an increase in the angle of attack normally results in a decrease in the rate of ascent and an increase in the rate of descent. Conversely a decrease in the angle of attack normally results in an increase in the rate of ascent and a decrease in the rate of descent. Consequently vertical departures from the glide path are continuously corrected through controlled changes in the angle of attack by means of a continuous automatic adjustment of power output while the attitude is held constant. The

\ ideal condition is one in which the angle of attack is such that the rate of descent of the aircraft corresponds with the rate at which the glide path is descending. It is important to note that the final value selected for attitude will be the attitude of the airplane ywhen contact is made with the runway and the angle of attack for holding the glide path in still air will be the sum of the angle of attitude and the angle of the glide path. It is therefore possible to select a predetermined 1 normal landing attitude, and if this is done the airplane will land on the runway without the necessity of manual control by the pilot and at a speed corresponding to the normal landing speed of the airplane. Direction down the runway will then be maintained by an automatic following of the localizer beam until the airplane has slowed down in its landing run at which point the pilot may take over the controls. It is also possible to select an attitude the angle of which is smaller than the angle of normal landing attitude. This gives a greater air speed for holding the same glide path but this is a desirable feature when the aircraft is Iwell out from the landing field.

Various procedures may be employed in using the present invention as will be apparent to those familiar with the known instrument approach art. I have found that the following procedure is particularly eiective. After crossing the eld in a direction corresponding to that of the outbound localizer beam at an altitude of about 2000 feet above the field, the aircraft localizer circuit is energized and the plane is therefore caused to hold the outbound localizer beam. At this point the circuit for holding the glide path is not activated but a constant predetermined angle of attack is maintained as a matter of convenience. After following the outbound localizer beam for a predetermined time sufficient to take the airplane a few miles away from the airport, the localizer circuit is disconnected and a 45 turn away from the localizer beam is made. The course away from the beam is held for a short period followed by a turn back to the localizer beam. On approaching the localizer beam, the localizer circuit is again connected which results in an automatic bracketing of the localizer beam on an inbound course at an altitude below that of the glide path. At this point the landing gear and iiaps are lowered. The attitude is then set to correspond with landing attitude. When the aircraft approaches the glide path as indicated on the cross pointer indicator, the glide path circuit is energized. The aircraft is then automatically brought to the runway by following the glide path through the action of the above described system. High intensity runway lights may be employed where it is intended that the pilot take over operation of the controls just prior to making contact with the runway.

13 In making a complete landing using the system of the present invention, the automatic controls are permitted to operate until the aircraft makes Acontact with the runway and slows down suiciently in the landing run. This is possible under conditions of little or no wind or where the wind direction is parallel to the runway since the landing track is positioned over the center of the run-way and has a relatively gradual descending slope. Where cross wind conditions exist it may be necessary to introduce an additional correction during the last part of the approach in order to avoid landing with excessive drift. Such correc-tions may be accomplished by correcting the heading just prior to making contact with the ground or by introducing a slip component surncient to counteract drift. Means for accomplishing this are known to those skilled in the art and therefore need not be described in this specification. As used in this specification and the appended claims the designation approach and landing system is intended to comprehend both complete landings as described above and landings in which manual control is vtaken over by the pilot a short time prior to contact being made with the runway.

In case of prolonged departures of the airplane above the glide path there is a possibility that the actual angle of attack as measured by pointer 9 will reach a point at which the airplane will stall. Other conditions such as reduced power in turns may lead to the same diiiiculty. In order to make certain that such a condition of stall cannot occur, provision is made for automatically and simultaneously increasing power, stopping the turn and depressing the elevators in the event that the airplane reaches a predetermined critical angle of attack. Each of these correcting actions works to prevent further approach to a stall and is made independently of the other control circuits. For instance, increase of power is accomplished by closing switch 'I4 and this has the effect of operating motor I4 in a direction to increase power thus tending to decrease angle of attack. As will be seen from Fig. 3 this action is independent of any action at power adjustor I3 which is the normal control for motor Id but does not occur if the airplane has landed and switch |05 has opened. This feature is necessary since the increase in angle of attack occurring as the airplane slowed down in the landing run would otherwise result in an increase in power. Decrease of turn is accomplished through causing motor IUS to operate when switch 'I9 closes. The connection to turn control is such that the action of motor I06 overrides any action which would normally be coming from localizer receiver I8. This may be done mechanically by an overriding clutch or electrically by automatically opening 'the circuit to localizer receiver I8 A part of the action of automatic pilot 2| in stopping the turn is to depress the elevators. In addition to this action in a turn, motor I22 is used to depress the nose if a critical angle of attack is reached when the airplane is in straight flight when switch 'I9 is closed. The closing of switch 'I9 is so arranged that it follows in point of time the closing of switch 14. On landing, switch I It is opened and this disconnects the motor circuits IUS and H22 and thus prevents undesired turn and dive corrections in the landing run which would otherwise result from operation of the emergency circuits.

The system of the present invention as described above operates to maintain the airplane provided ior manually operating power control.

I5 to hold the airplane on the glide `path. As shown in Fig. 2 a manual control 'I3 for adjusting power is provided. This manual control may consist of the standard throttle controls arranged to operate independently of the eilect of motor I4. This may be accomplished by an overriding clutch as is well known in this art. In the process of operating the system manually, the pilot holds the airplane on the glide path by adjusting the throttles through manual control 73 in such a way as to maintain pointer 9 in the same position as Ipointer I2. Since pointer 9 indicates the actual angle of attack of the airplane and Pointer I2 indicates the requested angle of attack set up through the action of motor 1, changes in power required to cause pointer 9 to follow changes in the position of pointer l2 will hold the airplane on the glide path. The pilot merely increases power when the requested angle of attack is less than the actualy angle of attack and decreases power when the requested angle of attack is greater than the actual angle of attack. There is thus provided an emergency procedure in the event of failure of a part of the automatic system.

Having described the use of my invention in carrying out approaches and landings, I will now describe its use as a system for accomplishing automatic flight control. In order to achieve maximnum efficiency in the operation of an airplane, particularly over relatively long distances where the fuel load is continuously decreasing, it is necessary to maintain a constant angle of attack. rFhis may be accomplished by the present invention by manual adjustment of variable resistance through manual control 55 which is mounted in such a way as to override the connection between motor l and movable arm 53. With motor l disconnected by opening switch I25, a positioning 0i movable arm 53 through manual control 55 results in the establishment of a requested angle o1" attack and this requested angle will be indicated by pointer I2 through a suitable mechanical linkage as shown in Fig. 2. Power adjustor I3 and motor I4 will then maintain the necessary amount of power through power control l5 to hold this requested angle of attack regardless of changes in load or attitude. With this angle of attack held constant, the desired altitude may be maintained or changed by making suitable adjustments in control 29 of automatic pilot 2l or by manually operating elevators 25, In either case the change in the position of elevators 25 will not aiect the angle of attack which will automatically be maintained at the requested Value through adjustments in the power output. However, such changes in the position of elevators 25 will result in a change oi attitude which will tend to cause the airplane to gain altitude, to lose altitude or to hold a constant altitude subject to variations due to rising or falling of masses of air. The desired altitude may therefore be maintained by changes in attitude. As is the case of the approach and landing system, the requested angle of attach established for flight control may be maintained automatically or by manual adjustment of the throttles to cause movable pointer 9 to coincide in position with pointer I2.

In describing the present invention specific means have been shown for accomplishing the desired results. It will be understood, however, that otherl means may be substituted for those shown without departing from the invention.

Thus, `a variety of other devices could be used for detecting and responding to changes in angle of attack. Likewise, the reversing network and speed network may be varied and need not be operatively or mechanically connected to the cross pointer indicator as shown in the drawings but could be connected to othel1 means responsive to the glide path receiver. Moreover, electrical circuits may be substituted for the mechanical elements shown without departing from the present invention.

Having thus described my invention, I claim:

1. In a flight control system for aircraft, means for maintaining an airplane at a predetermined attitude, manually adjustable selector means for establishing a value corresponding to a desired angle of attack, detector means for measuring the angle of attack of said airplane, and means responsive both to said detector means and said selector means forvarying the power output of said airplane to maintain the angle of attack at the value established by said selector means.

2. In an aircraft approach and landing system for automatically maintaining an airplane on a glide path defined by electromagnetic radiation, means for maintaining the attitude of said airplane at a predetermined angle, selector means responsive to vertical departures of the airplane from said glide path for establishing a value corresponding to an angle of attack for bringing said airplane back to said glide path, detector means responsive to changes in the angle of attack of the airplane for establishing a value corresponding to angle of attack, regulating means for varying the power output of the airplane, and control means responsive both to said selector means and to said detector means for controlling the operation of said regulating means to change the angle of attack of said airplane toward the value determined by said selector means.

3. In an aircraft approach and landing system for maintaining an airplane on a predetermined flight track defined by electromagnetic radiation, means responsive to said flight track and to changes in the angle of attack of said airplane for automatically positioning and maintaining said airplane on said flight track, said means including means responsive to lateral displacements of said airplane from said night track, means responsive to said lateral displacement means whereby said airplane is laterally maintained on said flight track, selector means esponsive to vertical displacements of said airplane from said flight track, angle of attack means responsive to changes in the angle of attack of said airplane, means for maintaining said airplane at a preselected attitude independently of changes in the angle of attack and means for varying the power output and the angle of attack of said airplane in response to the combined action of said selector means and said angle of attack means, whereby said airplane is positioned and maintained on said night track.

4. In an aircraft approach andlanding system for maintaining an airplane on a glide path defined by electromagnetic radiation, receiver means responsive to said glide path, selector means responsive to said receiver means for establishing a requested angle of attack, detector means responsive to changes in the angle of attack of the airplane, regulating means for varying the power output from the power unit of the airplane, and means responsive to the action of said selector means and said detector means 16 for controlling said regulating means whereby the angle of attack of said airplane is caused to vary to correspond with said requested angle of attack.

5. In an aircraft approach and landing system for maintaining an airplane on a glide path defined by electromagnetic radiation, receiver means responsive to said glide path, selector means responsive to said receiver means for establishing a requested angle of attack, detector means responsive to changes in the angle of attack of said airplane, regulating means for varying the power output from the power unit of the airplane, means responsive to the action of said selector means and said detector means for controlling said regulating means whereby the angle of attack of said airplane is caused to vary to correspond with said requested angle of attack, and alarm means responsive to a predetermined critical position of said detector means for automatically increasing the power output from said power unit when said airplane reaches a predetermined critical angle of attack.

6. In an aircraft approach and landing system for maintaining an airplane on a glide path defined by electromagnetic radiation, receiver means responsive to said glide path, selector means responsive to said receiver means for establishing a requested angle of attack, detector means responsive to changes in the angle of attack of the airplane, means for maintaining said airplane at a substantially constant instantaneous attitude, regulating means for varying the power output from the power unit of the airplane, means responsive to action of said selector means and said detector means for controlling said regulating means whereby the angle of attack of said airplane is caused to vary to correspond with said requested angle of attack, and alarm means responsive to a predetermined critical position of said detector means for automatically increasing the power output from said power unit and for automatically changing said attitude when said airplane reaches a critical angle of attack.

7. In an aircraft approach and landing systern for maintaining an airplane on a flight track at the intersection of a glide path beam and a iocalizer beam, the combination of glide path ,means and localizer means, said glide path means comprising selector means responsive to vertical departures of the airplane from said glide path, detector means responsive to changes in the angle of attack of the airplane, regulating means for varying the power output from the power unit of the airplane, and means for controlling the operation of said regulating means in response to said selector means and said detector means whereby said airplane is maintained on said glide path, and said localizer means comprising receiver means responsive to said localizer beam and automatic pilot means responsive to the output of said receiver means whereby said airplane is maintained on said localizer beam.

3. The invention of claim 2 further characterized by the fact that said selector means includes a speed network for varying the rate of correction of requested angle of attack with the extent of the vertical departure of the airplane from the glide path.

9. The invention of claim 2 further characterized by the fact that said selector means includes a reversing network for changing the direction of requested angle of attack change in the event of 17 rapid vertical movements of the airplane relative to the glide path.

10. The invention of claim 2 further characterized by a balancing network which includes elements operative in response to said selector means and said detector means for balancing said selector values and said detector values against each other and means for operating said regulating means in response to the difference between said values.

11. The invention of claim 2 further characterized by the fact that safety means are included responsive to said detector means for increasing power output at a predetermined critical angle of attack.

12. The invention of claim 2 further characterized by the fact that said regulating means comprise the throttle controls of said airplane.

13. The invention of claim 2 further characterized by the addition of means for deenergizing said control means when said airplane makes contact with the ground.

14. The invention of claim 7 further characterized by turn control means as an element of said automatic pilot means, and means responsive to said detector means for actuating said turn control means at a predetermined critical angle of attack of said airplane whereby a decrease in turn of said airplane is effected.

15. The invention of claim '7 further characterized by dive control means as an element of said automatic pilot means, and means responsive to said detector means for actuating said dive control means at a predetermined critical angle of attack of said airplane whereby the attitude angle of said airplane is reduced.

16. In an aircraft approach and landing system for automatically maintaining an airplane on a glide path defined by electromagnetic radiation, means for maintaining the attitude of said airplane at a predetermined angle, receiver means responsive to said glide path radiation for deriving a signal which varies with departures of the airplane from said glide path, means responsive to changes in the angle of attack of said airplane, means responsive both to said receiver means signal and to said angle of attack means for varying the power output of said airplane and for changing the angle of attack of said airplane to direct said airplane back to said glide path.

EDWIN F. SAXMAN, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,143,137 Basim et al Jan, 10, 1939 2,311,642 Crane et al Feb. 23, 1943 2,410,058 Frische et al Oct. 29, 1946 2,412,035 Dornak Dec. 3, 1946 2,423,336 Mosely July 1, 1947 

